1. Field of the Invention
The present invention relates to a process for spinning silk fibers. More specifically, the invention involves forming silk fibers by dissolving silk fibroin in an aqueous salt solution, removing the salt from the solution, followed by removal of the water, and redissolution of the resulting regenerated silk in hexafluoroisopropanol (HFIP) to produce a fiber-spinnable solution. The solution can be spun and drawn to produce high-quality fibers with near-native silk properties having greater mechanical strength.
2. Description of the Related Art
Silk fibroin (silkworm silk) is a naturally occurring polypeptide which occurs in fibrous form having high strength and a soft hand. The nature of silk fibroin makes it suitable for a wide range of uses including textile applications and in suture materials. Silk has been used as a suture material since ancient times. Because silkworms produce filaments in only one size (ca. 1 denier), twisted or braided yarns must be used when loads exceed a few grams. Unfortunately, the interstices of a multifilament yarn can be a route for infection. Thus, it would be desirable to be able to produce silk fibers in deniers other than those found in nature which would be suitable for such applications as monofilament sutures.
Fibroin is known to be soluble in certain high ionic strength aqueous salt solutions, for example, aqueous lithium thiocyanate (LiSCN), sodium thiocyanate (NaSCN), calcium thiocyanate (Ca(SCN).sub.2), magnesium thiocyanate (Mg(SCN).sub.2), calcium chloride (CaCl.sub.2), lithium bromide (LiBr), zinc chloride (ZnCl.sub.2), magnesium chloride (MgCl.sub.2), and copper salts, such as copper nitrate (Cu(NO.sub.3).sub.2), copper ethylene diamine (Cu(NH.sub.2 CH.sub.2 CH.sub.2 NH.sub.2).sub.2 (OH).sub.2), and Cu(NH.sub.3).sub.4 (OH).sub.2. It has long been known that the salts can be dialyzed out of such aqueous salt/fibroin solutions to produce aqueous solutions of fibroin which are similar in some ways to the liquid contents of a silkworm's silk gland. Fibers have been spun from aqueous fibroin solutions of this type, but more commonly, the solutions have been used to cast films for structure studies.
For example, Bhat and Ahirrao, Journal of Polymer Science Vol. 21, pp. 1273-1280 (1983) describe the dissolution of silk fibers in 70% lithium thiocyanate solution and regenerating the dissolved silk by casting films from the solution after dialyzing. They found that the cast films were amorphous and could o be transformed to a beta-sheet form using a variety of methods.
Those skilled in the art have attempted to find suitable solvents for preparing silk fibroin solutions which may be subsequently spun into fibers.
For example, Otoi et al., Japanese Kokoku Patent No. SHO 57[1982]-4723 describe a method for preparing a silk spinning solution involving dissolution of fibroin in an aqueous solution of copper-ethylenediamine, copper hydroxide-ammonia, copper hydroxide-alkali-glycerin, lithium bromide, sodium thiocyanate, or nitrates or thiocyanates of zinc, calcium, or magnesium. The solution is then dialyzed using a multilayered structure and used to fabricate fibers or films.
Bley, U.S. Pat. No. RE 22,650, discloses preparing fiber-spinnable polypeptide solutions containing a protein selected from the group consisting of silk fibroin, casein, gelatin, wool, and alginic acid in a solvent selected from quaternary benzyl-substituted ammonium bases.
U.S. Pat. No. 4,171,505 (Lock) describes a process for spinning polypeptide fibers including preparing fibers from a spinnable solution of silk fibroin in a solvent mixture of formic acid and lithium chloride.
Although it has been possible to produce silk fibroin fibers from such spinning solutions as described above, these solvents tend to be harsh and may degrade the fibroin. Dichloroacetic acid and trifluoroacetic acid are especially harsh and subject the polymer to a measurable degree of degradation. Fibers prepared from such solutions tend to be deficient in certain physical properties, such as mechanical strength.
Thus, a desirable solvent for preparing silk fibroin solutions is hexafluoroisopropanol (HFIP), because there is no detectable degradation of the fibroin in this solvent. However, in the past, it has not been possible to prepare silk fibers from HFIP solutions, since natural silk fibroin is not soluble in this solvent. Now, in accordance with this invention, a method for preparing silk fibroin fibers from silk fibroin/HFIP solutions has been discovered.